The present invention relates to a solid fuel burner and a method of combustion using the same for combusting a solid fuel carried by air flow; and in particular, to: a solid fuel burner and a method of combustion using the same which are applicable in a wide range of furnace loads and hence suitable for combusting a moisture-rich and volatile-matter-rich fuel such as pulverized coal, wood, and peat, and further permitting combustion by reducing the concentration of nitrogen oxides (NOx) in the exhaust gas; a combustion apparatus, such as a furnace, a heating furnace, and a hot blast stove, using said burner, and a method of operation thereof; a coal-fired boiler and a system using the same; and a coal-fired power generation system.
Pollution control regulations have been tightened in recent years for the purpose of environmental protection. In particular, for the above-mentioned kind of pulverized-coal boilers for combusting coal, reduction in the generation of NOx in the exhaust gas (NOx reduction, hereafter) is seriously required. Two-stage combustion methods are known as combustion techniques (NOx reduction techniques) for reducing the concentration of NOx generated in exhaust gas. The two-stage combustion methods are classified into the following two approaches. One approach is to reduce the NOx generation of a furnace as a whole, while the other approach is to reduce the NOx generation of a single burner. In the approach to reduce the NOx concentration of a furnace as a whole, the air ratio (ratio of the amount of supplied air to the amount of necessary air for completely combusting an amount of fuel; the air ratio of unity corresponds to one stoichiometric equivalent) in the burner zone of the furnace is maintained below unity. In this fuel-rich condition, generated NOx is chemically reduced, and hence NOx reduction is achieved. The unburnt carbon resulting from this approach is completely combusted with air added through an air inlet provided downstream of the burner zone.
In the approach to reduce the NOx generation of a single solid fuel burner (simply a burner, in some cases hereafter) such as a pulverized-coal burner, secondary and tertiary air flows are swirled, thereby delaying the mixing thereof with the flow of pulverized-coal burning with primary air alone. By virtue of this, a large chemical reduction region is formed (such a burner is called a NOx-reduced burner, hereafter). This approach is implemented in a NOx reduction pulverized-coal burner (Japanese Unexamined Patent Publications Nos. Sho-60-176315 and Sho-62-172105).
These techniques have achieved a reduction of NOx concentration in the exhaust gas down to 130 ppm (fuel ratio=fixed carbon/volatile matter=2, nitrogen content in the coal=1.5%, and unburnt carbon content in the ash=5% or less). Nevertheless, the regulated value of NOx concentration in the exhaust gas has been tightened year by year, and the required value of NOx concentration in the exhaust gas for the near future is 100 ppm or less.
NOx-reduced burners capable of reducing NOx generation down to 100 ppm or less have been developed. Such burners include: a burner having an internal flame stabilizing ring for reinforcing the NOx-reduced combustion in the burner section; and a burner having a flame stabilizing ring for bridging between an internal flame stabilizing ring as described above and an external flame stabilizing ring provided in the outer periphery of the combustion nozzle through which the mixture of pulverized coal and carrier gas flows.
By the way, in geological areas where an increase in energy demand is expected, a majority in the near future will use low-quality coal which is rich in moisture and ash matter and has a low calorific value. Among various low-quality coals, high-moisture content coal, such as brown coal and subbituminous coal, is found in abundance. Nevertheless, such coal has a problem of poor fuel performance, such as a lower flame temperature and poor combustibility, in comparison with bituminous coal. Brown coal is found mainly in Eastern Europe, and is a rather young coal containing 20% or more ash matter and 30% or more moisture.
Further, low-coalification coal (such as brown coal and lignite), wood, and peat are rich in volatile matter which is released in gaseous form while being heated, and are also rich in moisture. Such kinds of fuel have a lower calorific value than that of high-coalification coal such as bituminous coal and anthracite, and in addition, are generally poor in pulverizability. Further, the ash of such kinds of fuel has a lower melting point. The richness in volatile matter easily causes spontaneous ignition during storage and pulverization processes in air. This causes difficulty in handling processes in comparison with the case of bituminous coal and the like. In order to avoid this difficulty, when brown coal and lignite are pulverized and combusted, a mixture of exhaust gas and air is used as the fuel carrier gas. Since the mixture gas has a lower oxygen concentration, the spontaneous ignition of fuel is prevented. Further, the residual heat in the exhaust gas helps evaporation of moisture in the fuel carried by the mixture gas.
Nevertheless, since the fuel is carried by the low-oxygen-concentration gas, the combustion reaction does not proceed until the fuel ejected from the burner is mixed with air. That is, the combustion reaction is limited by the rapidity of mixing of the fuel with air. This causes a slower combustion rate than that of bituminous coal which can be carried by air. Accordingly, the time necessary to burn out is longer than that of bituminous coal. This causes an increase in unburnt carbon in the furnace exit.
A method for accelerating the ignition of the fuel carried by the carrier gas of low oxygen concentration is to provide an air ejection nozzle in the tip of the fuel nozzle so as to increase the oxygen concentration of the fuel carrier gas. For example, Japanese Unexamined Patent Publication No. Hei-10-73208 discloses a burner having an air nozzle outside a fuel nozzle. Further, commonly used is a burner having an air nozzle at the center of a fuel nozzle so as to accelerate the mixing of the fuel with air at the exit of the fuel nozzle.
Further, Japanese Unexamined Patent Publication No. Hei-4-214102 discloses a burner comprising: a fuel nozzle for ejecting a mixture of pulverized coal and carrier gas; and a secondary air nozzle and a tertiary nozzle provided outside the fuel nozzle; where in a flame stabilizing ring for maintaining the flame obtained by the pulverized coal ejected from the fuel nozzle is provided at the tip of a septum between the fuel nozzle and the secondary air nozzle.
As described above, brown coal is an inexpensive fuel. Nevertheless, its characteristics of a high ash content, a high moisture content, and a low calorific value cause problems in combustibility and ash cohesion. As for combustibility, the key technology to efficient combustion depends on how to accelerate the ignition and form a stable flame. The ash cohesion to the burner structure and the furnace wall surface is caused by the low melting point of the ash. This is because the brown coal is rich in calcium, sodium, and the like. Further, the ash cohesion is accelerated by the fact that the brown coal needs to be supplied in a larger amount in order to compensate the lower calorific value in comparison with the bituminous coal, thereby generating a larger amount of ash. Such slagging and fouling is a disadvantage of the brown coal. Accordingly, in order to use low-quality coal, such as brown coal and lignite, for burner combustion, both efficient combustion and ash cohesion reduction need to be achieved.
Methods for combusting brown coal generally used abroad are a tangential firing method and a corner firing method. In the former method, a burner compartment composed of fuel passages and combustion air passages is provided in each sidewall of a furnace. In the latter method, a burner compartment composed of fuel passages and combustion air passages is provided in each corner of a furnace.
Described below is the difference in these methods from a so-called opposed firing method in which a group of burners are provided in each of the opposing wall surfaces of a furnace as is generally used in Japan for combusting bituminous coal.
In the opposed firing method, each burner (multiple tubes for fuel and combustion air) is operated by a self flame stabilizing method. In the methods for combusting brown coal, instead of the self flame stabilizing method implemented in the exit of the burner, a jet of combustion air has momentum, and is mixed with fuel around the center of the furnace, thereby causing a stable combustion.
FIG. 30 is a front view of an example of a burner compartment 37 according to the corner firing method or the tangential firing method, viewed from the furnace side. Each airflow has a diverse velocity depending on its diverse purpose. A center air nozzle 124 mixes air into the fuel supplied by an exhaust gas flow from a fuel nozzle 125, thereby increasing oxygen concentration and accelerating the combustion. An outermost air nozzle 126 supplies a highly penetrative jet having a velocity of 50 m/s or higher, thereby stabilizing the fuel combustion around the center of the furnace.
A key technology necessary to establish dominance within the world market in the relatively new field of combustion of low-quality coal such as brown coal is a pulverized-coal burner operable even in load variation depending on the variation in electric power demand. In Eastern Europe, boilers need to operate at a partial load as low as 30% in some cases. In such cases, the prior art has the following problems.
As described above, the important point in the prior art combustion of brown coal (corner firing method and tangential firing method) is to provide a highly penetrative jet of the mixture of fuel and combustion air, so as to stabilize combustion in the furnace. By decreasing the load in furnace, the above-mentioned momentum of the jet from the burner compartment 37 also decreases, thereby causing instability in the flame. FIG. 31 is a cross sectional plan view of a furnace 41 according to a corner firing method, showing an example of the variation of flame shape when the load of the furnace 41 decreases from a high-load state to a low-load state. At a high load as shown in FIG. 31(a), the jet from the burner compartment 37 forms a blow-off section 38 near the exit of the burner, and further forms a stable combustion region between the vicinity of the exit and the center of the furnace 41, thereby achieving efficient combustion.
In contrast, at a low load, the flow velocity and hence the momentum of each jet from the burner compartment 37 decreases. Accordingly, a stable combustion region as shown in FIG. 31(a) is not formed, and hence the combustion is unstable (the whole region of the furnace 41 becomes dark as shown in FIG. 31(b)). For the purpose of prevention of the flame extinction of the burner at a low load, a flame detector 48 for monitoring the formation of a stable combustion region in the furnace 41 is provided near an after-air port 49 in the upper part of the furnace 41 as shown in the cross sectional views of FIGS. 32. This flame detector 48 determines that flame extinction has occurred when the brightness of the furnace 41 decreases as shown in FIG. 31(b).
As such, since the formation of a stable combustion region in the furnace 41 is affected by the momentum of the jet of each burner, the prior art method is not applicable at a low load. Here, in FIGS. 31(a) and 31(b), burner compartments 37 are provided in the lower part of the furnace 41, whereby the jets of the mixture of fuel and carrier gas from the burner compartments 37 are mixed with combustion air supplied from the after-air ports 49, thereby forming a flame.
Further, in the high-load operation of a prior art combustion apparatus (furnace), that is, when a large amount of fuel is supplied to the burners, heat radiation from the flame heats up the burner structure to a higher temperature. Since the ash of low-coalification coal such as brown coal and lignite has a lower melting point, the ash lying on the heated-up section of the burner structure melts down, and the fused ash grows gradually. The grown fused ash can disturb the combustion of the fuel. Accordingly, in the high-load operation, the flame needs to be formed in a position far from the burner.
An object of the invention is to provide: a solid fuel burner and a method of combustion using the same which can perform stable combustion in a wide range of furnace loads from a high-load operation condition to a low-load operation condition and hence are suitable for combusting low-coalification fuel such as brown coal and lignite; a combustion apparatus using said burner, and a method of operation thereof; and a coal-fired boiler using said burner.
Another object of the invention is to provide: a burner suitable for the opposed firing method and capable of efficiently combusting pulverized coal, such as brown coal having poor ash characteristics, near the exit of the burner and thereby preventing ash cohesion in the vicinity of the burner; and a combustion apparatus using the same.
Another object of the invention is to provide: a burner suitable for the corner firing and the tangential firing method and capable of forming a stable combustion region around the center of a furnace even at a low combustion load of the furnace by preventing ash cohesion onto the furnace side walls; and a combustion apparatus using the same.
Another object of the invention is to provide: a multi-tube burner suitable for the opposed firing method and corner firing and the tangential firing method; and a combustion apparatus using the same.
Further, the NOx-reduced burner according to the prior art has a configuration suitable for reducing the NOx concentration in the exhaust gas in the combustion of ordinary bituminous coal. Nevertheless, in a combustion apparatus using highly ignitable fuel such as brown coal and peat, the carrier gas used is not primary air but the exhaust gas of low oxygen concentration for the purpose of preventing spontaneous ignition. In this case, ignition near the burner is difficult and causes the following two problems.
(1) Because of difficulty in flame piloting near the burner, operation without assisted combustion by oil or fuel gas is limited to a high load condition in which the combustion temperature in the combustion furnace is sufficiently high.
(2) NOx reduction is not achieved, because the combustion rate is low in the near-the-burner region where the fuel is in excess over the combustion air, that is, because the combustion rate is high after mixing with secondary and tertiary air.
The above-mentioned problems result from use of a low-oxygen concentration gas as the coal carrier gas. In order to resolve the problems, it may be possible to supply combustion air into the fuel nozzle near the exit of the burner so as to increase the oxygen concentration. Nevertheless, this configuration reduces the concentration of the pulverized coal, and hence does not improve the ignition performance.
Thus, an object of the invention is to provide a solid fuel burner capable of rapidly and efficiently combusting pulverized coal, such as brown coal having poor ash characteristics, near the exit of the burner and thereby achieving NOx-reduced combustion; and a combustion apparatus using the same.
A burner according to the invention is suitable for the use of a mixture fluid consisting of: a solid fuel composed of low-coalification coal such as brown coal and lignite; and a carrier gas having an oxygen concentration less than 21%.
(1) A first burner according to the invention is a solid fuel burner comprising: a center air nozzle for ejecting air; a fuel nozzle provided outside the center air nozzle and ejecting a mixture fluid consisting of a solid fuel and a carrier gas; additional air holes or additional air nozzles provided in the inner wall surface of the fuel nozzle and ejecting air; and one or more outer air nozzles provided outside the fuel nozzle and ejecting combustion air.
In the above-mentioned burner, it is possible to increase the amount of air ejected from the additional air holes or additional air nozzles for ejecting air along the inner side of wall of the fuel nozzle. The air ejected from the additional air holes or additional air nozzles increases the oxygen concentration near inner side of wall of the fuel nozzle. This accelerates the combustion in comparison with the case of a lower oxygen concentration. Accordingly, ignition of the fuel quickens, and hence a flame is formed starting from the vicinity of the fuel nozzle.
In an above-mentioned burner further comprising a swirler within the center air nozzle, the method of ejecting air from the center air nozzle can be selected, in response to the combustion load, from the two methods of: (1) a straightforward jet or weakly swirled jet; and (2) a strongly swirled jet.
In this case, (a) the exit of the center air nozzle and/or (b) the exit of the additional air holes or additional air nozzles are preferably located upstream of the exit of the fuel nozzle inside the burner. According to this configuration, mixing of the fuel with the air ejected from (a) the center air nozzle and/or (b) the additional air holes or additional air nozzles is achieved within the fuel nozzle. This permits a partial increase in oxygen concentration of the fuel carrier gas.
Both the distance between the exit of the fuel nozzle and the exit of the center air nozzle and the distance between the exit of the fuel nozzle and the exit of the additional air holes or additional air nozzles are set preferably so that the residence time of the fuel in the fuel nozzle does not exceed the ignition delay time (approximately 0.1 s) of the fuel. The purpose of this is to prevent a back fire and burn damage caused by the flame formation within the fuel nozzle. Since the fuel carrier gas generally flows through the fuel nozzle at a velocity of 10-20 m/s, the above-mentioned distances are 1-2 m or less.
When a passage contracting member for temporarily contracting the cross section of the fuel nozzle gradually starting from the upstream of the burner and for recovering the cross section is provided in the inner side of wall of the fuel nozzle of the burner, the flow of fuel particles (of pulverized coal) having a larger inertia than that of the fuel carrier gas (exhaust gas and the like) is focused to the center axis region. Further, when a condenser consisting of both a conical section having a gradually increasing cross section starting from the upstream of the burner and a subsequent conical section having a gradually decreasing cross section is provided on the outer side of wall of the center air nozzle so as to be located downstream of the passage contracting member, the flow of fuel particles (of pulverized coal) having been focused at the center axis region is expanded by the condenser and then flows through the passage of the fuel nozzle after passing through the condenser. At that time, the flow of fuel particles (of pulverized coal) having a larger inertia than that of the fuel carrier gas (exhaust gas) is concentrated in the inner side region of wall of the fuel nozzle, and flows directed towards the exit. This concentrated flow of pulverized coal in the inner-wall side region of the fuel nozzle easily contacts with outer air (combustion air) near the exit of the fuel nozzle, and further contacts with the high-temperature gas of a recirculation zones generated downstream of the flame stabilizing ring described later, thereby being ignited easily.
When the above-mentioned burner according to the invention is operated at a high load, the fuel ejected from the fuel nozzle is heated up by a strong thermal radiation from the furnace. This situation permits stable combustion even when the fuel is ejected from the fuel nozzle at a high velocity. At that time, air is ejected as a straightforward jet or weakly swirled jet (at a swirl number of 0.3 or lower) from the center air nozzle, whereby the flame is blown off from the vicinity of the burner so that the flame is formed in a position far from the burner. This prevents high temperatures from heating up the burner structure by thermal radiation of the flame.
In contrast, when the above-mentioned burner according to the invention is operated at a low load, air is ejected as a strongly swirled jet (at a swirl number of 0.5 or higher) from the center air nozzle, whereby the mixing of the fuel jet with air is accelerated. Further, since the ejection velocity of the fuel at the burner center axis is reduced by the swirled air jet from the center air nozzle, the residence time of the fuel near the fuel nozzle becomes longer. Accordingly, the fuel is heated up near the fuel nozzle at a temperature necessary for combustion, whereby a flame is formed starting from the vicinity of the fuel nozzle.
In the above-mentioned burner according to the invention, the ratio between the amount of air ejected from the center air nozzle and the amount of air ejected from the additional air holes or additional air nozzles is adjustable depending on the combustion load. For example, at a low combustion load, the amount of air ejected from the center air nozzle is reduced, whereas the amount of air ejected from the additional air holes or additional air nozzles is increased. In contrast, at a high combustion load, the amount of air ejected from the center air nozzle is increased, whereas the amount of air ejected from the additional air holes or additional air nozzles is reduced.
In the above-mentioned burner according to the invention, the amount of air is adjusted during combustion preferably so that the air ratio to volatile matter (the ratio of the total amount of air supplied from the fuel nozzle, the center air nozzle, and the additional air holes or additional air nozzles, to the amount of air necessary for the complete combustion of the volatile matter contained in the fuel) is 0.85-0.95.
Further, an obstacle (flame stabilizing ring) against both the gas flow from the fuel nozzle and the air flow from the outer air nozzle is preferably provided at the tip of a septum between the fuel nozzle and the outer air nozzle.
The pressure downstream of the flame stabilizing ring decreases, whereby a recirculation zones directing from the downstream to the upstream is formed. Within the recirculation zones, burnt gas of high temperature generated in the downstream resides in addition to the fuel and air ejected from the outer nozzles. Accordingly, the recirculation zones is at a high temperature, thereby serving as an ignition source for the fuel jet flowing in the vicinity. This permits the stable formation of a flame starting from the exit of the fuel nozzle.
Further, a flame stabilizing ring having a shark-tooth shaped protrusion may be provided on the inner wall surface of the exit of the fuel nozzle. Such a flame stabilizing ring similarly accelerates the ignition of the fuel.
In the above-mentioned burner according to the invention, the cross section of the downstream passage of the center air nozzle may be smaller than the cross section of the upstream passage of the center air nozzle, and then the position of the swirler provided within the center air nozzle may be movable in the burner center axis direction within the center air nozzle. According to this configuration, the adjustment of the position of the swirler permits the adjustment of the intensity of the air flow swirl depending on the combustion load.
At a low load, the swirler is moved to the downstream having a smaller cross section in the center air nozzle, whereby the air jet from the center air nozzle is strongly swirled, thereby forming a flame near the burner. In contrast, at a high load, the swirler is moved to the upstream having a larger cross section in the center air nozzle, whereby the air jet from the center air nozzle is weakly swirled, thereby forming a flame in a position far from the burner within the furnace.
When the temperature of the burner or the furnace wall surface rises too high, burnt ash coheres onto the burner structure and furnace wall, and the cohesion grows gradually. This phenomenon called slagging tends to occur. In order to suppress the slagging, in response to a signal from a thermometer or a radiation intensity meter provided in the burner or the furnace wall surface, the amount or the swirl intensity of air ejected from the center air nozzle may be adjusted, or alternatively, the amount of air ejected from the additional air holes or additional air nozzles may be adjusted. The adjustment of the amount or the swirl intensity of air causes a change in the position of flame formation within the furnace, thereby permitting the adjustment of the intensity of thermal radiation to the burner and the furnace wall surface.
At a high load, because of a high thermal load of the furnace, the flame is preferably formed in a position far from the burner. At a low load, because of a lower thermal load of the furnace, the temperature of the burner and the furnace wall surface does not rise as high in comparison with the case of the high load even when the flame is formed near the burner.
When the above-mentioned burner according to the invention is used in such a combustion apparatus, the center air nozzle has a cylindrical shape. A pair of air tubes for supplying air are connected to a part upstream of the center air nozzle. Each of the air tubes is connected so as to introduce air from a tangential direction at a mutually opposing position of the circular cross section of the center air nozzle. When the combustion apparatus is operated at a high combustion load (for example, 60-70% or higher), each air tube supplies an identical amount of air into the center air nozzle. In contrast, when the combustion apparatus is operated at a low combustion load (for example, 60-70% or lower), each air tube supplies a diverse amount of air into the center air nozzle. By virtue of such operation, the swirl intensity of the center air jet is adjusted depending on the load.
(2) A second burner according to the invention is a solid fuel burner comprising: a fuel nozzle for ejecting a mixture fluid consisting of a solid fuel and a carrier gas; additional air holes or additional air nozzles provided inside the wall surface of the fuel nozzle and ejecting air; and one or more outer air nozzles provided outside the wall surface of the fuel nozzle and ejecting air. In contrast to the first burner, the second burner does not comprise a center air nozzle for ejecting air.
In the second burner according to the invention, it is possible to increase the amount of air ejected from the additional air holes or additional air nozzles for ejecting air along the inner wall surface of the fuel nozzle. The air ejected from the additional air holes or additional air nozzles increases the oxygen concentration near the inner wall surface of the fuel nozzle. This accelerates the combustion reaction of the fuel in comparison with the case of a lower oxygen concentration. Accordingly, ignition of the fuel quickens, and hence a flame is formed starting from the vicinity of the fuel nozzle.
In the above-mentioned burner, the exit (tip) of the additional air holes or additional air nozzles is preferably located upstream of the exit (tip) of the fuel nozzle within the burner. According to this configuration, mixing of the fuel with the air ejected from the additional air holes or additional air nozzles is achieved within the fuel nozzle. This permits a partial increase in oxygen concentration of the fuel carrier gas. The distance between the exit of the fuel nozzle and the exit of the additional air holes or additional air nozzles is set preferably so that the residence time of the fuel in the fuel nozzle does not exceed the ignition delay time (approximately 0.1 s) of the fuel. The purpose of this is to prevent a back fire and burn damage caused by the flame formation within the fuel nozzle. Since the fuel carrier gas generally flows through the fuel nozzle at a velocity of 10-20 m/s, the above-mentioned distance is 1-2 m or less.
A passage contracting member for temporarily contracting the cross section of the fuel nozzle gradually starting from the upstream of the burner to the downstream direction and for recovering the cross section is preferably provided inside of the wall surface of the fuel nozzle of the burner. This contraction of the passage cross section accelerates of the fuel nozzle the velocity of the fuel carrier gas flowing through the fuel nozzle. Accordingly, even when a flame is formed within the fuel nozzle due to a temporary slow down of fuel flow, a back fire is prevented from proceeding upstream of the contracted passage part formed by the passage contracting member. Further, when a condenser consisting of both a section having a gradually increasing cross section starting from the upstream of the burner to the downstream direction and a subsequent section having a gradually decreasing cross section is provided inside the fuel nozzle so as to be located downstream of the passage contracting member, the flow of fuel particles (of pulverized coal) having been focused at the center axis region is expanded by the condenser and then flows through the passage of the fuel nozzle. At that time, the flow of fuel particles (of pulverized coal) having a larger inertia than that of the fuel carrier gas is concentrated in the inner side of wall of the fuel nozzle, and flows directed towards the exit. This concentrated flow of pulverized coal in the inner side of wall of the fuel nozzle easily contacts with the outer air ejected from the outer air nozzle, in the vicinity of the exit of the fuel nozzle. The flow of pulverized coal further contacts with the high-temperature gas of a recirculation zones generated downstream of a flame stabilizing ring described later, thereby being ignited easily.
Further, a flame stabilizing ring opposing both the flow of the solid fuel mixture from the fuel nozzle and the flow of air is preferably provided at the tip of a wall surface between the fuel nozzle and the outer air nozzle.
The pressure in the furnace downstream of the flame stabilizing ring decreases, whereby a recirculation zones of the mixture fluid directing from the downstream to the upstream is formed. Within the recirculation zones, burnt gas of a high temperature generated in the region downstream of the burner in the furnace resides in addition to air, the fuel and fuel carrier gas ejected from the fuel nozzle and the outer air nozzle. Accordingly, the recirculation zones is at a high temperature, thereby serving as an ignition source for the fuel jet. This permits stable formation of a flame starting from the exit of the fuel nozzle.
A flame stabilizing ring having shark-tooth shaped protrusions may be provided inside the wall surface of the tip (exit) of the fuel nozzle. Such a flame stabilizing ring similarly accelerates the ignition of the fuel.
When exhaust gas is used as the solid fuel carrier gas, exit of additional air holes or additional air nozzles is provided between the conical section having a decreasing cross section in the condenser and the flame stabilizing ring. This configuration permits the mixture gas to have amounts of oxygen necessary for combustion. This mixture gas collides with the flame stabilizing ring, thereby permitting efficient ignition by the flame stabilizing ring. Further, even at a low combustion load and even when the furnace combusts pulverized coal such as brown coal having poor ash characteristics, rapid and effective combustion is achieved near the exit of the burner, thereby permitting NOx-reduced combustion and preventing ash cohesion in the furnace wall surface near the burner.
In the above-mentioned burner according to the invention, the amount of air ejected from the additional air holes or additional air nozzles can be adjusted depending on the combustion load of the combustion apparatus (furnace).
Generally in solid fuel burners not restricted to the above-mentioned solid fuel burner according to the invention, at a high combustion load of the combustion apparatus (furnace), the flame of the solid fuel is preferably formed in a position far from the solid fuel burner within the furnace. In contrast, at a low combustion load of the combustion apparatus (furnace), the flame of the solid fuel is preferably formed starting from the vicinity of the furnace wall surface immediately downstream of the exit of the fuel nozzle for solid fuel.
For example, when additional air holes or additional air nozzles are provided in a solid fuel burner, at a low combustion load of the combustion apparatus (furnace), it is possible to increase the amount of air ejected from the additional air holes or additional air nozzles. The air ejected from the additional air holes or additional air nozzles increases the oxygen concentration near the inner wall surface of the fuel nozzle. This accelerates the combustion reaction of the fuel in comparison with the case of a lower oxygen concentration. Accordingly, ignition of the fuel quickens, and hence a flame is formed starting from the vicinity of the exit (tip) of the fuel nozzle. At a high combustion load of the combustion apparatus (furnace), the amount of air ejected is decreased from the additional air holes or additional air nozzles. This operation decreases the oxygen concentration near the inner side of wall of the fuel nozzle, thereby decelerating the combustion reaction of the fuel in comparison with the case of a lower oxygen concentration. Accordingly, ignition of the fuel becomes slow, and hence a flame is formed in a position far from the burner within the furnace.
At a high combustion load of the combustion apparatus (furnace), the temperature of the solid fuel burner and the furnace wall surface rises. Accordingly, burnt ash coheres onto the burner structure, and the cohesion grows gradually. This phenomenon called slagging tends to occur. In order to suppress the slagging onto the burner structure and the furnace wall surface, at a high combustion load of the combustion apparatus (furnace), the flame is moved to a position far from the burner, thereby comparatively reducing the temperature of the burner and the furnace wall surface. At a low combustion load, the amount of air is adjusted preferably so that the air ratio to volatile matter (the ratio of the total amount of air supplied from the fuel nozzle and the additional air holes or additional air nozzles (if any), to the amount of air necessary for the complete combustion of the volatile matter contained in the fuel) is at 0.85-0.95. Stable combustion is generally difficult to achieve at a low combustion load. However, the air ratio to volatile matter at 0.85-0.95 raises the flame temperature, thereby permitting the continuation of stable combustion.
Further, in order to suppress the slagging onto the burner structure and the furnace wall surface, in response to a signal from a thermometer or a radiation intensity meter provided in the burner or the peripheral furnace wall surface, the amount of air ejected from the additional air holes or additional air nozzles may be adjusted. The adjustment in the amount of air causes a change in the position of flame formation within the furnace, thereby permitting the adjustment of the intensity of thermal radiation to the burner and the furnace wall surface.
As described above, at a high load of the combustion apparatus, because of a higher thermal load of the furnace, the flame is preferably formed in a position far from the burner within the furnace. In contrast, at a low load of the combustion apparatus, because of a lower thermal load of the furnace, the temperature of the burner and the peripheral furnace wall surface does not rise as high in comparison with the case of high load. Accordingly, the flame may be formed near the burner within the furnace.
In the method of combustion using the first burner or the second burner, at a high load of the combustion apparatus, the fuel is ignited in a position far from the burner, and hence the flame is formed in the center of the furnace. In order to monitor the flame generated by the burners at a high load, the flame is monitored preferably at the center of the furnace where the flame at the burners merges. In contrast, at a low load of the combustion apparatus, the fuel is ignited near the burner, and the flame is formed near each burner. Further, in some cases, an independent flame is formed by each burner within the furnace. Accordingly, the flame formed in the exit of each burner is preferably monitored at a low load.
In the first burner and the second burner according to the invention, additional air holes may be used in place of additional air nozzles. The additional air holes are provided in the wall surface of the fuel nozzle, and have the shape of a circle, ellipse, rectangle, or square. Four, eight, or twenty or so, at maximum, of additional air holes may be provided in equal spacing in the radial direction of the fuel nozzle. A single additional air hole formed by a slit in the radial direction of the fuel nozzle disadvantageously causes a non-uniform flow of the additional air ejected from the slit, within the fuel nozzle.
Preferably, heated air is supplied to the additional air holes or additional air nozzles. The heat source for this purpose maybe the pressurized air supplied to the fan mill for pulverized coal generation, or alternatively, the air provided to the air box heated for combustion in the burner. The pressurized air supplied to the fan mill is more preferable because of its rather high pressure.
The air supplying section for the additional air holes or additional air nozzles may be connected to an air box for supplying combustion air (outer air such as secondary and tertiary air) to the outer air nozzle. However, it is more preferable that the air supplying section is connected to a dedicated combustion-air supplying apparatus for supplying combustion air.
When the air supplying section for the additional air holes or additional air nozzles is connected to the dedicated combustion-air supplying apparatus, oxygen-enriched air having an enriched oxygen concentration or pure oxygen can be easily supplied depending on the combustibility of the solid fuel such as pulverized coal and in response to a reduction in the load of the combustion apparatus. Further, a combustion air flow rate regulating apparatus provided for a dedicated combustion-air supplying apparatus permits easy control of the supply rate.
Further, when combustion gas (air) effective for the ignition of the fuel is supplied to the burner by a dedicated combustion-air supplying apparatus, the pressure of the combustion gas (air) can be different from that obtained by the air box. This permits unrestricted selection of the aperture size of the supply of the combustion gas for ignition. Furthermore, a combustion air flow rate regulating apparatus provided for the dedicated combustion-air supplying apparatus permits easy control of the supply rate.
A guide for defining the direction of outer air ejection is provided in the exit of the outer air nozzle of the above-mentioned first burner and second burner according to the invention, whereby the flow of outer air (secondary and tertiary air, in some cases hereafter) is provided with a certain divergence so as to form a divergent flame. At that time, the inclination of the guide is set to be 45 degrees or less relative to the burner center axis, whereby a momentum is provided to the combustion air jet ejected from the outer air nozzle so as to involve the mixture fluid of exhaust gas and pulverized coal. The flame is narrowed by the air jet having a larger momentum, whereby a stable flame (combustion region) is formed in the furnace, thereby permitting efficient combustion of the pulverized coal.
When the guide for guiding the outermost air jet from the outermost air nozzle is provided at such an angle that the outermost air jet moves along the burner and the outside furnace wall surface, the outermost air jet cools down the burner and the outside furnace wall surface, thereby preventing slagging.
Combustion apparatuses equipped with a plurality of above-mentioned first burner and second burner according to the invention in the furnace wall surface include a coal-fired boiler, a peat-fired boiler, and a biomass-fired (wood-fired) boiler and a heating furnace and a hot blast stove.
Thermometers or radiation intensity meters are provided in the above-mentioned first burner and second burner according to the invention or the furnace wall outside the burner. In response to a signal from such an instrument, the amount and the swirl intensity of air ejected from the center air nozzle of the burner are adjusted, and/or the amount of air ejected from the additional air holes or additional air nozzles is adjusted. By virtue of this operation, the position of flame formation in the furnace is appropriately controlled depending on the load change.
An example of the appropriate position measurement of the flame formation is as follows. At a low combustion load of the combustion apparatus, the tip of in-furnace flame of the solid fuel is formed near the furnace wall outside the exit of the fuel nozzle. At a high combustion load of the combustion apparatus, the flame is formed starting from a position far from the exit of the fuel nozzle by 0.5 m or more on the center axis of the fuel nozzle.
At a high load of the combustion apparatus, the flame is monitored around the center of the furnace where the flame at the burners according to the invention merges, by using a flame detector or by visual inspection. In contrast, at a low load of the combustion apparatus, the flame generated at the exit of each burner according to the invention is monitored.
The invention includes a coal-fired boiler system and a coal-fired power generation system described below.
(a) A coal-fired boiler system comprising: a coal-fired boiler; a flue for serving as a passage for the exhaust gas from the boiler; an exhaust-gas cleaning apparatus provided in the flue; a pulverized-coal carrying apparatus for carrying the coal in the form of pulverized coal to the burner according to the invention provided in the boiler; a pulverized-coal supply adjusting apparatus for adjusting the amount of pulverized coal supplied from the pulverized-coal carrying apparatus to the burner; and an air supply adjusting apparatus for adjusting the amount of air ejected from the burner.
(b) In a coal-fired power generation system comprising: a furnace having a furnace wall equipped with a plurality of burners according to the invention; a boiler for boiling the water to generate steam by using the combustion heat obtained by combustion of solid fuel by the burners; a steam turbine driven by the steam obtained by the boiler; and a power generator driven by the steam turbine; a coal-fired power generation system using solid fuel burners according to the invention as said burners.
In the first and the second burner according to the invention, a burner according to the corner firing method or tangential firing method which has been difficult to operate at a low load of a furnace in the prior art is operated in a scheme such that a stable flame combustion region is formed around the center of the furnace at a high load, and is operated in a self flame stabilizing scheme at a low load.
At that time, a unit is formed by solid fuel burners according to the invention, and a plurality of units are arranged at the corners or in opposing sidewalls of the furnace so as to form a pair or pairs of units.
This method of combustion is applicable to a wide range of furnace loads (specifically 30-100%) corresponding to the power demand variation even in a furnace for combusting low-quality coal such as brown coal and lignite.
Specifically, at a high load, a blow-off section is formed at the bottom of the fuel jet from the burner. At a low load, a self flame stabilizing scheme is used, that is, combustion is carried out starting from the bottom of the fuel jet from the burner. The blow-off or the ignition at the bottom of the fuel jet from the burner is controlled by adjustment of the distribution ratio of the combustion air (outer air and outer most air) of the burner and/or by the adjustment of the swirl intensity of the combustion air by using a swirler provided in the outer air nozzle of the burner.
When the burner according to the invention is applied to a boiler furnace, the boiler can be operated depending on the power demand. This prevents the over-generation of steam for the electric power in the boiler furnace, thereby permitting efficient operation of the boiler furnace and substantially reducing the running cost of the boiler furnace.